1. Field of the Invention
The present invention relates to a protection relay in which an influence of distortion component of fault current generated by a fault in a power system is suppressed.
2. Description of the Related Art
Generally, a protection relay is used to monitor a power system. A main technical subject of such a protection relay is to reduce an influence of harmonics in fault current and fault voltage generated in a fault in a system contained in a signal inputted from the system.
Particularly because charging capacity of a system is increased in underground power transmission cable, phase modifying capacitor and the like, the order of generated harmonics tends to be lowered.
Thus, if it is intended to secure a desired damping amount according to a conventional method for damping the harmonic component with a digital filter, filter delay time needs to be prolonged, so that relay operation time is delayed.
For the reason, an approximation method, which is not affected by harmonic theoretically even if such harmonics is contained has been employed in recent years.
An example of such approximation method will be described with reference to FIG. 1. Power transmission impedance constant up to a fault point F will be considered with reference to FIG. 1. Voltage and current of a protection relay installation point A are assumed to be v and i when resistance is R and inductance is L. If it is assumed that the voltage at the fault point F is zero, differential equation of a power transmission line 2 can be expressed in an expression (1).V=R·I+L·(di/dt)  (1)
By calculating a differential item (di/dt) of the expression (1) approximately, detection accuracy can be improved even if harmonic is not removed with a filter. An example of a concrete method for digital calculation actually adapted is shown below.vm+vm−1=R·(im+im−1)+L·(im−im−1)vm−1+vm−2=R·(im−1+im−2)+L·(im−1−im−2)   (2)
When a reactance value X (=ω0·L) is calculated from the expression (2), the inductance is expressed in expression (3).                               L          m                =                                            X              m                        /                          ω              0                                =                                                                      (                                                            v                      m                                        +                                          v                      m                                        -                    1                                    )                                ·                                  (                                                            i                                              m                        -                        1                                                              +                                          i                                              m                        -                        2                                                                              )                                            -                                                (                                                            v                                              m                        -                        1                                                              +                                          v                                              m                        -                        2                                                                              )                                ·                                  (                                                            i                      m                                        +                                          i                                              m                        -                        1                                                                              )                                                                                                      (                                                            i                      m                                        -                                          i                                              m                        -                        1                                                                              )                                ·                                  (                                                            i                                              m                        -                        1                                                              +                                          i                                              m                        -                        2                                                                              )                                            -                                                (                                                            i                                              m                        -                        1                                                              -                                          i                                              m                        -                        2                                                                              )                                ·                                  (                                                            i                      m                                        +                                          i                                              m                        -                        1                                                                              )                                                                                        (        3        )            
Lm/L (true value) is as expressed in the expression (4) under conditions of the expressions (5) and (6), so that frequency characteristic of Xm/X (true value) is as indicated with the dotted curve of FIG. 2.Lm/L(true value)=tan(ω0T/2)/tan(ωT/2)  (4)
Where, im=Isin(ωtm) vm=Vsin(ωt+θ)im−im−1=2Isin(ωT/2)cos(ωtm−ωT/2)  (5)vm+vm−1=2Vcos(ωT/2)sin(ωtm−ωT/2+θ)  (6)
Therefore, frequency characteristic of Xm/X (true value) is as indicated with the dotted curve of FIG. 2.
In FIG. 2, its abscissa axis indicates frequency (order) and its ordinate axis indicates a reactance measurement value when basic frequency of system electrical quantity is 50 Hz. Further, in FIG. 2, its dotted curve indicates a case of sampling at 600 Hz and its solid line indicates a case of sampling at 4800 Hz.
As indicated in FIG. 2, the value of Lm/L (true value) decreases below 1 as the frequency departs from its fundamental wave. FIG. 2 indicates that the value of (ωT/2) only should be suppressed to substantially 1 (that is, the sampling period should be set small) when this value (Lm/L) is near twice or three times the fundamental wave.
Frequency characteristic when the sampling frequency is actually multiplied eight times is indicated by the solid line of FIG. 2. Qauntitavely, the relation between an approximate amount (im−im−1) of the differential item and a differentiated amount (vm+vm−1) is indicated by the expressions (7) and (8). Therefore, if the sampling frequency is raised (the period is decreased), approximation accuracy of the differential item can be raised.sin(ωT/2)=ωT/2 cos(ωT/2)=1im−im−1=2I·sin(ωT/2)·cos(ωtm−ωT/2)=2I·ωT/2·cos(ωtm−ωT/2)  (7)vm+vm−1=2V·cos(ωT/2)·sin(ωtm−ωT/2+θ)=2V·sin(ωtm−ωT/2θ)  (8)
However, the value of the expression (7) is a very small value with respect to an amplitude value I. Therefore, a relative value of noise (quantization error generated at the time of A/D conversion, white noise generated in an analog circuit) contained in sampling data (im, im−1) is increased thereby disabling practical use of this method.
For example, when the sampling period T is T=1/4800 sec, ω0=2π·50 Hz, the second item ε/(ω0T/2) on the right side of the expression (9) is amplified to about 30 times. An e in the expression (9) is noise error.
 (im−im−1)/(ω0T/2)=2I·(ω/ω0)·cos(ωtm−ωT/2)+ε/(ω0T/2)  (9)